Established patterning methods used in the thick film industry include printing (screen or other), spraying, brushing, spin coating, dipping, laminating, photo-patterning and etching. Screen-printing is by far the most commonly used method for patterning thick film circuits. A certain amount of a thick film composition is placed at the edge of a printing screen wherein a squeegee moves and presses the thick film composition across the screen. A circuit pattern similar to the screen openings is printed on the substrate. The printed substrate is then dried which causes most of the solvent in the thick film composition to evaporate, and then fired which causes burnout of any organic vehicle. Although screen print pattern resolution can be improved by optimizing screen wire-mesh dimensions, emulsion type, thickness, and screen printing variables; there is an accepted opinion in the industry that screen printed fine line and space resolution is not adequate for certain applications, since screen printed line resolutions are about 125 microns or larger.
Photo-patterning and etching approaches are alternative patterning technologies that offer uniform finer lines and space resolution when compared to screen-printing. A photo-patterning method, such as DuPont's FODEL® printing system, utilizes a photosensitive organic medium as found in patents U.S. Pat. Nos. 4,912,019; 4,925,771; 5,032,490, whereby the substrate is first completely covered (printed, sprayed, coated or laminated) with the photosensitive thick film composition and dried. An image of the circuit pattern is exposed by actinic radiation through a photomask bearing a circuit pattern, the exposed substrate is then developed. The unexposed portion of the circuit pattern is washed away leaving a patterned substrate that is fired to remove all remaining organic materials and sinter inorganic materials. Such a photo-patterning method demonstrates thick film line resolution of about 30 microns or larger depending on the substrate smoothness, inorganic particle size distribution, exposure and development variables. The narrow window of particle size distribution, circuit line conductivity, thickness and circuit line thickness uniformity are limitations of this thick film photo-patterning method.
Another system of patterning is an etching method, a thick film composition is printed, dried and fired on a substrate surface. A photoresist is then applied on top of the fired thick film surface that is sequentially exposed and developed. The substrate is then dipped or sprayed with a chemical that etches the fired metallization in the open areas of the photoresist. After washing and resist removal, a desired circuit pattern is produced on the substrate. Line capabilities of etch patterning is about 10 microns or larger depending on the substrate smoothness, particle size distribution of the inorganic powders, type and activity of the etching solution and photoresist adhesion. The use of hazardous chemical etching solutions and remnant inorganic binder found on top of the substrate surface are some limitations of the thick film etch patterning approach for display and other applications.
Yet, another system is described in U.S. Pat. Nos. 5,110,384, 5,167,989 and 5,296,259. These patents describe a dry powder method of applying conductive or phosphor particles in a pattern to a substrate. Precleaned ceramic substrates are coated with a thin layer of a phototackifiable material, exposed imagewise with UV light, toned with metal powder toner material and fired according to conventional thick film firing profiles.
The graphic arts industry widely uses photosensitive elements to create color patterns of an image. The elements usually comprise a support, a photosensitive layer and a cover sheet. The photosensitive layer of the element either looses tackiness or becomes tacky upon exposure to actinic radiation. The exposure of the photosensitive layer is used to create a latent image which is then formed by applying toner particles by way of a powder or a toner sheet to the imagewise exposed element. The toner particles stick to the tacky areas so that an image is created by the toner particles adhered to the tacky areas. U.S. Pat. No. 3,649,268 describes a reproduction process using positive-working photosensitive elements. Negative-working photosensitive elements are also known which require a dual exposure to provide a negative image with tacky and non-tacky areas. Negative-working elements and their process of use are disclosed in U.S. Pat. Nos. 4,174,216 and 4,247,619.
The present invention combines the concept of forming an imaged pattern on a substrate using a photosensitive element used in the graphics art industry and a thick film composition used in established methods of pattern formation in the electronics industry. The invention does not include the step of developing a photosensitive element that is needed to form color images in the graphics art industry. It has been found that the invention produces high quality circuit patterns with substantially no shrinkage, edge curl or undercut at a low cost in an environmentally compatible manner.